The British Army was at the centre of new torture allegations last night after pictures showed an Iraqi prisoner being battered with rifle butts, threatened with execution and urinated on by British soldiers.

The shocking images drew immediate condemnation from the prime minister and led the Ministry of Defence to launch an investigation.

The prisoner - thought to have been a thief - had his jaw broken and his teeth smashed during an eight-hour ordeal after being arrested near the southern Iraqi city of Basra.

Bleeding and vomiting, he was eventually driven away from the army camp, still hooded, and thrown off the back of a moving vehicle. He was not charged with an offence and it is not known whether he lived or died.

The photographs were given to the Mirror newspaper by soldiers who said the horrific treatment of prisoners was widespread and was one of the main reasons why coalition forces faced such fierce opposition in Iraq.

One told the newspaper: "We are not helping ourselves out there. We are never going to get them on our side. We are fighting a losing war."

Last night the chief of general staff, General Sir Michael Jackson, condemned the incident.

Sir Michael said: "I am aware of the allegations which have been made today of the abuse of prisoners by British soldiers in Iraq. If proven, not only is such appalling conduct clearly unlawful, but it also contravenes the British Army's high standards.

"All allegations are already under investigation. If proven, the perpetrators are not fit to wear the Queen's uniform. They have besmirched the good name of the Army and its honour.

"Most emphatically, the British Army should not be judged by the reprehensible ill-discipline of a few soldiers who, by their shameful behaviour, have let down those tens of thousands of British soldiers who have worked, and still do, in difficult and dangerous conditions in the most commendable way, in particular in Iraq, where their sole purpose is to help the Iraqis to a new and better future."

A soldier who witnessed the man's arrest and beating told the Mirror how the prisoner had been held during a raid on suspected thieves at Basra docks in southern Iraq.

"We just caught the one guy that time. Straight away, he gets a beating - a couple of punches and kicks to put him down. Then he was dragged to the back of the vehicle." A sandbag was placed over the man's head and his hands tied.

The soldier said the man had been hit with batons. "You normally try to leave off the face until you are in camp," he told the Mirror. "If you pull up with black eyes and bleeding faces, you could be in a bit of shit. So it is body shots, just scaring him."

The prisoner was kept for around eight hours while the beatings were carried out. The photographs show him being bludgeoned with rifle butts in the head and groin.

A gunbarrel is placed in the prisoner's mouth through the bag on his head, while others kick and stamp on him. One soldier urinates on him.

In the final few photographs the man is barely conscious, his shirt torn, while vomit seeps through the sack on his head.

The soldier, who admits he took part in the attack, said the man was pleading with the others to stop.

"He could speak a few words, 'No mister, no mister.' What I did was less than others, but yes, I joined in." The soldier said he feared the man would die. "He was dying, basically, so he could not take any more, so basically they threw him out."

"One of the officers came down to get him and it was like, a bit of a mini-bollocking, but nothing really. Then it was, 'Get rid of him, I've not seen him. The paperwork gets ripped."